Gas cushion near or around perforating gun to control wellbore pressure transients

ABSTRACT

A method for perforating a formation material proximate to a wellbore involving lowering a perforating gun string comprising a perforating gun and a gas-generating device downhole, providing wellbore fluid around the perforating gun, and providing a volume of gas proximate the perforating gun, by the gas-generating device, wherein the volume of gas is configured to reduce the shock produced upon firing of the perforating gun.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application 61/357457, filed on Jun. 22, 2010.

FIELD OF THE INVENTION

The present invention relates to a gas cushion surrounding a perforating gun in a wellbore, for purposes of controlling wellbore fluid compressibility and wellbore pressure transients.

BACKGROUND

After drilling, oil wells are typically protected with steel casing that is secured to the wellbore with cement. To produce well fluid (oil or gas) from a hydrocarbon bearing formation, the formation is typically perforated from a wellbore to enhance fluid communication between the reservoir and the wellbore. Perforation is the process that creates a direct link between the wellbore and a producing formation by puncturing holes through the casing and the cement sheath that surrounds it. Perforation guns carrying shaped charges are typically used for this purpose. These shaped charges contain explosives. When the explosives are fired, they produce high pressure and high temperature. As a result, the shaped charge liners are collapsed and shot out as jets, which can penetrate the casing and the nearby formation.

Firing of perforating guns into reservoir rocks, however, may cause sudden pressure changes to wellbore pressure transients, which may include the wellbore pressure, the reservoir pressure, and the perforating gun pressure. Such sudden pressure changes typically accompany the perforating process, and often cause damage to the formation surrounding the wellbore and/or to the perforation system. As drilling depths increase, controlling the sudden changes to wellbore pressure transients has become an increasing concern.

SUMMARY

In general, in one aspect, the invention relates to a method for perforating a formation material proximate to a wellbore involving lowering a perforating gun string comprising a perforating gun and a gas-generating device downhole, providing wellbore fluid around the perforating gun, and providing a volume of gas proximate the perforating gun, by the gas-generating device, wherein the volume of gas is configured to reduce the shock produced upon firing of the perforating gun.

In general, in one aspect, the invention relates to a perforating gun system for perforating a formation material proximate to a wellbore, comprising a perforating string comprising a perforating gun, wherein the perforating string is lowered into the wellbore; and a gas-generating device operatively connected to the perforating string and configured to introduce a volume of gas proximate to the perforating gun, wherein the volume of gas is configured to reduce the shock produced upon firing of the perforating gun.

Other aspects of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a perforating gun system in a wellbore in accordance with one or more embodiments of the invention.

FIG. 2 shows a perforating gun system with gas cushion near the gun in accordance with one or more embodiments of the invention.

FIG. 3 shows a graph of pressure data compared for two different perforating gun systems in accordance with one or more embodiments of the invention.

FIG. 4 shows a perforating gun system surrounded by gas bubbles in accordance with one or more embodiments of the invention.

FIG. 5 shows a perforating gun system completely surrounded by a gas cushion in accordance with one or more embodiments of the invention.

FIG. 6 shows a graph of pressure data corresponding to the perforating gun system of FIG. 5 in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION

Specific embodiments of the invention will now be described in detail with reference to the accompanying figures Like elements in the various figures are denoted by like reference numerals for consistency.

In the following detailed description of embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description.

In general, embodiments of the invention provide a method and apparatus for creating a gas “cushion” in the vicinity of a perforating gun system lowered into a wellbore, either before or just at the time or perforation. More specifically, the purpose of the gas cushion is to increase the apparent compressibility of the wellbore fluid in the vicinity of the perforating gun. The effect of this increased compressibility is to reduce both the magnitude and rate of any sudden pressure changes which accompany the perforating process. Controlling wellbore compressibility, and by extension the magnitude and rate of pressure change in the wellbore, is useful to accomplish one or more of the following desirable objectives:

-   -   a. minimize transient loading on gun string and completions         equipment (i.e. “gun shock”);     -   b. minimize initial perforation tunnel and/or wellbore collapse         and sand production; and     -   c. maintain a static underbalance condition which has been set         prior to perforating.

FIG. 1 shows, during a typical downhole operation, a long and tubular device, such as a perforating gun 15, is run into the wellbore 11 in preparation for production. After the perforating gun 15 has been deployed at its appropriate downhole position on a line 17 or tubing, e.g., wireline, e-line, slickline, coiled tubing, etc., perforating charges are fired. Typical perforating charges, such as shaped charges 14, may be radially disposed and outwardly directed toward the region of the formation rock to be perforated. Firing these shaped charges 14, may generate extremely high-pressure jets 28, which can produce perforation tunnels 18 through both casing 12 (if the wellbore is cased) and surrounding rock formation 13. The skin 19 of the tunnel 18 may then be made more permeable for releasing its hydrocarbons from the formation 13.

In one or more embodiments of the invention, the perforating system described above may include a gas generating device 30. The gas generating device 30 may be displaced below the perforating gun 15, and operatively connected to the perforating string used to lower the perforating gun 15 into the wellbore. The gas generating device 30 is configured to introduce a “gas cushion,” or an accumulation of gas, in proximity to the perforating gun some time before perforation occurs. For example, in one or more embodiments of the invention, the gas generating device 30 may be a propellant device, a high-pressure tank, a stored gas accumulator, a slow bladder inflator, a liquid CO₂ generating cylinder device, or similar device capable of generating a cushion of gas in proximity to the perforating gun. In one or more embodiments of the invention, the gas generating device may be retained under a packer.

In one or more embodiments of the invention, the amount of gas generated by the gas generating device 30 is proportional to the size of the perforating gun. That is, the range of volume of gas generated by the device 30 is a function of the length of perforating interval of the perforating gun. For example, the gas generating device 30 may generate a certain volume of gas for every foot diameter of the perforating gun.

In one or more embodiments of the invention, the gas generating device 30 may be actuated by a control signal sent from the surface to a circuit, valve, or other component of the gas generating device. Alternatively, the gas generating device 30 may be automatically actuated by conditions encountered as the device is lowered into the wellbore. For example, the gas generating device 30 may be depth actuated once the device reaches a predetermined depth in the wellbore. Similarly, the gas generating device could also be temperature actuated.

Those skilled in the art will appreciate that exactly when the gas generating device begins to introduce gas into the region surrounding the perforating gun may depend on the type of gas generating device employed and the method of actuation of the device. For example, a slow bladder inflator may be depth actuated and may begin to introduce gas minutes before perforation occurs. Alternatively, in one or more embodiments, a propellant generator device may actuate immediately before perforation occurs (e.g., via a control signal sent from the surface), which would result in introduction of gas 100^(ths) of milliseconds before perforation occurs. Further, those skilled in the art will appreciate that although the gas-generating device is show in FIG. 1 as being below the perforating gun and operatively connected to the gun string, the invention is not limited to such a configuration, as other alternative arrangements for the gas-generating device may be employed. For example, in an alternative embodiment of the invention, the gas cushion may be pumped downhole from the surface, in which case a gas-generating device located downhole may not be necessary. Instead, the gas-generating device may be a stand-alone device positioned on the surface.

The purpose of introducing an accumulation of gas into the area surrounding the perforating gun is to increase system fluid compressibility. The pressure differential between a wellbore and the reservoir prior to perforation may be described as under-balanced, balanced or over-balanced. A desirable under-balance condition exists when hydrostatic pressure inside the well casing is less than pressure in the formation. The term “underbalance” (UB) means pressure of the wellbore (Pwb)<pressure of the reservoir (Pres), and “overbalance” (OB) means Pwb>Pres. The terms “static UB” or “static OB” are now used when referring to the pressure condition prior to perforating because on the sub-second time scale, local pressure conditions can vary greatly from the initial static condition. These terms indicate the static pressure conditions which existed prior to perforating, and were assumed to remain throughout the perforating process. Exposing the newly-created perforation tunnels to a sufficiently large UB induces tensile and shear stresses within the perforation damaged zone; maintaining this UB throughout the perforating process induces a strong surge flow from the reservoir into the perforation tunnel and wellbore. These events are recognized as essential conditions to effectively remove perforation damage.

When a hollow carrier perforating gun is detonated, the equilibrium pressure inside the gun (a few milliseconds after charge detonation) plays an important role. If this pressure is less than the wellbore pressure (Pgun<Pwb), then this is called a relative “gun UB”; wellbore fluid gets sucked into the gun and Pwb drops. If Pwb even briefly drops to a value less than Pres, then a dynamic underbalance (DUB) condition is present. If Pgun>Pwb, this is called “gun OB”; detonation gas exits the gun and Pwb increases. If Pwb even briefly increases to greater than Pres, then a dynamic overbalance (DOB) condition is present. Those skilled in the art will appreciate that DUB or DOB conditions can be created from either initial static UB or OB conditions, and that the initial static condition alone (Pwb, Pres) does not predict the transient conditions which may occur during perforating.

In addition to the initial pressures involved and described above, a primary parameter governing wellbore pressure transients is the wellbore fluid compressibility. If the fluid is stiff, then pressure changes are large and rapid; if the fluid is compliant, then pressure changes are small and gradual. For example, consider the case of gun filling. The “initial” (i.e., before reservoir or far-field wellbore storage effects come into play) pressure drop in the wellbore fluid is represented as,

dP=dV/(B*V0),

where dV is the volume of fluid which enters the gun, V0 is the total wellbore fluid volume influenced by gun filling, and B is the wellbore fluid compressibility (inverse of bulk modulus or stiffness). Furthermore, the rate of pressure drop is represented as,

dP/dt=d[V/(B*V0)]/dt

dP/dt˜(dV/dt)/(B*V0)

So, for a given system (V0) and given volumetric rate of gun filling (dV/dt), the above relationships show that both the rate and magnitude of pressure drop are inversely proportional to the fluid compressibility B.

Accordingly, embodiments of the invention provide a gas-generating device for generating a gas cushion near the perforating gun inside a wellbore configuration, to increase overall system compressibility to reduce to rate and magnitude of pressure drop that occurs after perforation. Specifically, embodiments of the invention involve a method for perforating a wellbore, in which the following steps may occur. Initially, a perforating gun string including a perforating gun and a gas-generating device may be lowered into a wellbore. Next, wellbore fluid may be provided around the perforating gun. Finally, a volume of gas may be provided, proximate the perforating gun. The volume of gas is configured to reduce the shock produced upon firing of the perforating gun. Specifically, the volume of gas may be introduced by actuating the gas-generating device, either by a control signal or automatically by conditions encountered during lowering of the perforating gun string into the wellbore. Once actuated, the gas-generating device may slowly or quickly (depending on the type of device employed) introduce a volume of gas physically near the perforating gun. After the perforating gun is detonated, i.e., the shape charges are detonated and the gun perforates the wellbore casing, the volume of gas may rupture or vent/escape into the wellbore, eventually making its way back up to the surface.

FIGS. 2-6 show example perforating systems after the method described above takes place. As such, FIGS. 2-6 show a perforating system with a gas cushion accumulated nearby, and accompanying laboratory testing statistics for such perforating systems, in accordance with one or more embodiments of the invention.

Specifically, FIG. 2 shows a perforating gun-in-wellbore configuration 200, with a gas cushion 202 nearby, in accordance with one or more embodiments of the invention. FIG. 2 shows the various transient pressures including the pressure inside the perforating gun 206, the pressure in the wellbore 204, and the pressure of the reservoir 208. The gas cushion 202 may be created by a gas-generating device such as that described above with respect to FIG. 1.

In one or more embodiments of the invention, the gas cushion created by the gas-generating device, or the liquid/gas interface, may be considered “nearby” when the characteristic wave speed transit time is very short relative to the time scale of gun filling (or gun expulsion, as the case may be). In other words, the wellbore may be considered as two series springs—a stiff spring (liquid) backed by the soft spring (gas). As long as the liquid-gas interface is “close enough” to the perforating gun string, then the pressure response will be dominated by the gas.

FIG. 3 shows an example laboratory experiment demonstrating the magnitude- and rate-reducing effect on DUB of increased wellbore fluid compressibility, for the gun filling case corresponding to the arrangement of FIG. 2 described above. Differences between traces 302 and 304 were (a) gun volume, and (b) wellbore fluid compressibility. The trace graphed as 302 had a smaller gun volume and gas cushion (i.e., liquid/gas interface several inches away from gun). Both the smaller gun volume and increased wellbore fluid compressibility contributed to the modest pressure drop of only ˜500 psi of trace 302. Had the gun volumes been the same for tests conducted in both cases, the pressure of the 302 trace would have dropped by approximately ˜2000 psi, instead of by only ˜500 psi. But this pressure drop (and the rate) would still be far less than for the testing conducted in a stiff fluid system (very low fluid compressibility) exhibited by trace 304.

In one or more embodiments of the invention, in addition to generating a gas cushion-on-liquid, there may be other ways to increase wellbore fluid compressibility. For example, as shown in FIG. 4, gas may be bubbled around the perforating gun. In this case, the gas bubbles 402 may be introduced by a standard gas generator (not shown), or may be bubbling up from a lower producing hydrocarbon bearing formation. Alternatively, other techniques can be used to increase wellbore fluid compressibility, such as the addition of hollow microspheres.

In yet another alternative embodiment, gas may be placed everywhere around the gun 502, as shown in FIG. 5. Laboratory tests have demonstrated that the configuration shown in FIG. 5 has the potential to virtually eliminate the wellbore pressure transients altogether. FIG. 6 shows a graph of the pressure transients when a perforating gun is completely or substantially completely surrounded by a gas cushion as shown in FIG. 5. As can be seen in FIG. 6 from the flat curve 600, no pressure drops occur in such a system, because the gas cushion completely envelopes the perforating system, and eliminates all sudden changes in wellbore pressure transients.

Although FIG. 6 shows a balanced case, those skilled in the art will appreciate that an initial static underbalance may easily be created, which would remain throughout the perforating event until full recovery is brought about by some combination of reservoir inflow and farfield wellbore storage effects.

Embodiments of the invention described above may be utilized in many different scenarios of the perforating process. The below discussion provides examples for using embodiments of the invention described above. Those skilled in the art will appreciate that while particular perforating scenarios are provided as examples, these are not meant to limit the scope of the invention.

Typically, for perforation cleanup, large magnitude rapid pressure drops are generally desirable, and so usually require a very stiff fluid in the vicinity of the perforating gun. However, there are instances where the exact opposite effect is desired, i.e., either a modest pressure drop, or a very modest pressure increase, accompanied by a gradual rate. These cases call for a highly compressible wellbore fluid. For example, for some situations which may produce gun filling and DUB, a very modest (and gradual) pressure drop may be ideal, such as:

-   -   a. when perforating very weak formations, in order to minimize         the tendency of tunnel failure and/or excessive sand production;     -   b. when trying to minimize the net force loading (due to         pressure differentials) on the gun string and completion, in         order to reduce the tendency of damaged equipment, unseated         packers, tangled cables, etc.

Furthermore, there may be some situations in which it is desired to:

-   -   c. maintain, during the perforating event, an initial static UB         condition (i.e., avoid the DOB which can accompany a static UB         job, especially in situations where DUB is very difficult to         achieve—such as low reservoir pressures and/or very high shot         densities), in order to obtain clean perforations.     -   d. minimize the transient loading and hence damage on the         gunstring, conveyance device, and completion equipment (as b.         above)

All of the above scenarios can be addressed by employing very compressible fluid in the vicinity of the perforating gun. In the gun filling cases, the modest DUB will translate to reduced net loading (and hence displacement) of the gunstring and completion equipment (packers, tubing, etc.). Also, the reduced magnitude and rate DUB reduces the pore pressure gradient within the formation, which will in turn reduce the stresses on the formation (tensile, shear, and drag force). These may reduce the tendency of both tunnel collapse and transient sand production. In the case of openhole (OH) perforating, this may also effectively reduce the tendency to collapse the wellbore (if that were to be considered a potential problem).

For the gun OB cases, moderating the increase in Pwb from an initial static UB condition may allow one to preserve an initial static UB, and hence produce clean perforations in accordance with conventional “static UB” assumptions (pre-PURE; pre-early 2000's). An initial static UB of, say 2000 psi, may revert (with a very stiff fluid) to a DOB of a few hundred psi, and hence produce dirty perforations. With a compliant fluid, on the other hand, the 2000 psi static UB may be reduced to a 1500 psi transient UB, which may still yield fairly clean perforations.

The preceding discussions have focused on hollow carrier guns, wherein the shaped charges are isolated from the wellbore fluid by the carrier tube. However, embodiments of the present invention can also be applied in the case of exposed, or capsule charge perforating systems. These systems typically produce large overpressures in the wellbore, due to the direct coupling of the detonating charges and the wellbore fluid. The present invention, by increasing compressibility of the fluid surrounding the perforating system, minimizes the magnitude and rate of these dynamic overpressures. Accordingly, any attendant damage to casing, formation, or other downhole equipment would be minimized.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

1. A method for perforating a formation material proximate to a wellbore, comprising: lowering a perforating gun string comprising a perforating gun and a gas-generating device downhole; providing wellbore fluid around the perforating gun; and providing a volume of gas proximate the perforating gun, by the gas-generating device, wherein the volume of gas is configured to reduce the shock produced upon firing of the perforating gun.
 2. The method of claim 1, wherein firing the perforating gun comprises: detonating a plurality of shaped charges in the perforating gun.
 3. The method of claim 1, wherein the gas-generating device is positioned below the perforating gun on the perforating gun string.
 4. The method of claim 3, further comprising: actuating the gas-generating device, wherein the gas-generating device is actuated by one selected from a group consisting of: automatically, during lowering of the perforating gun downhole, and manually, using a control signal sent from a surface device to the gas-generating device.
 5. The method of claim 3, wherein the gas-generating device is one selected from a group consisting of a propellant device, a high-pressure tank, a stored gas accumulator, a liquid CO₂ generating cylinder device, and a slow bladder inflator.
 6. The method of claim 1, wherein the volume of gas comprises gas bubbles surrounding the perforating gun.
 7. The method of claim 1, wherein the volume of gas is an accumulation positioned directly above the perforating gun.
 8. The method of claim 1, wherein the volume of gas completely surrounds the perforating gun.
 9. The method of claim 1, wherein the volume of gas provided is proportional to a size of the perforating gun.
 10. The method of claim 1, wherein the volume of gas is configured to increase fluid compressibility in the wellbore, resulting in reducing sudden changes to wellbore pressure transients.
 11. The method of claim 1, wherein the volume of gas is introduced by the gas-generating device immediately before detonation of the perforating gun.
 12. A perforating gun system for perforating a formation material proximate to a wellbore, comprising: a perforating string comprising a perforating gun, wherein the perforating string is lowered into the wellbore; and a gas-generating device operatively connected to the perforating string and configured to introduce a volume of gas proximate to the perforating gun, wherein the volume of gas is configured to reduce the shock produced upon firing of the perforating gun.
 13. The perforating gun system of claim 12, wherein the gas-generating device is positioned below the perforating gun on the perforating gun string.
 14. The perforating gun system of claim 12, wherein the gas-generating device is one selected from a group consisting of a propellant device, a high-pressure tank, a stored gas accumulator, a liquid CO₂ generating cylinder device, and a slow bladder inflator.
 15. The perforating gun system of claim 12, wherein the volume of gas is introduced when the gas-generating device is actuated by one selected from a group consisting of: automatically, during lowering of the perforating gun string downhole, and manually, using a control signal sent from a surface device to the gas-generating device.
 16. The perforating gun system of claim 12, wherein the volume of gas completely surrounds the perforating gun.
 17. The perforating gun system of claim 12, wherein the volume of gas provided is proportional to a size of the perforating gun.
 18. The perforating gun system of claim 12, wherein the volume of gas is configured to increase fluid compressibility in the wellbore, resulting in reducing sudden changes to wellbore pressure transients. 